A double monochromator serves to obtain a higher spectral resolution with a given slit width than a single monochromator. In addition, the stray light is very much decreased in a double monochromator.
Single monochromators comprise a light source unit with a light source and a concave mirror, by which the light source is imaged onto the inlet slit of the proper monochromator. Light of different wave lengths, a continuum, for example, impinges upon the inlet slit in such a single monochromator. This light does not run only along the ideal theoretical path of rays through the apparatus, whereby only a defined spectral band would pass through the outlet slit. In fact, a certain portion of the light, having entered through the inlet slit, also passes to the outlet slit by other routes through scattering or the like, such that a component of other undesirable wave lengths also appears besides the desired spectral band at the outlet slit. Indeed, this component can possible be small. But the ratio of useful signal to interfering signal can deteriorate in an intolerable way due to unfavorable lamp sensitivity characteristics of many photoelectric receivers, which have a steeply declining flank at the edge of the scanned wave length range.
Essentially, only light of the desired wave length passes through the outlet slit of the monochromator on the entrance side in a double monochromator. The stray light is subjected to a new dispersion in the second monochromator on the outlet side, such that again only a certain fraction of this stray light passes to the outlet slit through scattering or the like. But this component of stray light is, however, very small after passing through a double monochromator. In addition, a new dispersion of the useful light occurs, such that the spectral resolution of the monochromator is improved with a given slit width.
German Published Patent Application No. 3,511,676, published on Oct. 3, 1985, cognate with U.S. Pat. No. 4,697,924, granted Oct. 6, 1987, describes four constructions of a double monochromator containing an entrance slit which is imaged in the plane of a median slit by first dispersing means, and an exit slit as well as second dispersing means, by means of which the median slit is imaged in the plane of the exit slit. This double monochromator also can be operated as a single monochromator. For this purpose, the first dispersing means are arranged at a rotatable base conjointly with a mirror which may be a concave mirror or a planar mirror, in a manner such that, for the double monochromator function, the first dispersing means and, for the single monochromator function, the mirror is respectively pivoted into the path of rays of the double monochromator. In the latter case, the mirror is placed precedingly of the median slit as viewed in the radiative direction.
In this known construction, the mirror replaces the first dispersing means in the path of rays for the single monochromator function. Consequently, the pivoting movement of the deflecting mirror requires also pivoting the dispersing means and decoupling the same from the associated wavelength drive means.
German Published Patent Application No. 3,640,044, published Jun. 1, 1988, relates to a monochromator arrangement including a single monochromator in connection with a path of rays, for example, of an adjustable infrared laser. The path of rays extends transversely relative to the monochromator through a deflection assembly housing containing an entrance diaphragm and an exit diaphragm. First and second deflecting mirrors are located therebetween and are arranged perpendicular to each other. The second deflecting mirror is transversely offset from the first deflecting mirror with respect to the path of rays. The first deflecting mirror deflects the incoming beam into the monochromator which directs an exit beam to further deflecting mirrors arranged following the exit diaphragm for passing the exit beam which has been deflected at the second deflecting mirror, to the path of rays leading to a detector.
From German Published Patent Application No. 3,443,727, published Jun. 12, 1986, there is known a microphotometer for image and wavelength scanning. Therein, a measuring diaphragm is displaceable, for scanning purposes, transversely relative to a path of rays leading to a detector and containing a lens. Optical deflection means can be inserted into the path of rays and contain mirrors which deflect the incoming light onto a path of rays extending through a single monochromator for wavelength scanning. This arrangement utilizes two displaceably arranged deflecting mirrors in order to direct light away from the predetermined path of rays and through the single monochromator back onto the path of rays.
U.S. Pat. No. 4,102,576, granted Jul. 25, 1978, relates to a double monochromator containing an entrance slit which is imaged in the plane of a median slit by first dispersing means, and an exit slit as well as second dispersing means by means of which the median slit is imaged in the plane of the exit slit. The slits are defined by respective slit jaws formed at two slit plates which are displaceable relative to each other by means of a wedge. In this manner, there can be adjusted an optimum exit band width for each double monochromator configuration.
German Patent No. 2,730,613, published on Jul. 26, 1979, relates to a double monochromator containing an input monochromator including a diffraction grating and a main monochromator including a diffraction grating. The input monochromator images a light source at the entrance slit of the main monochromator. A filter wheel is located between the light source and the input monochromator. The diffraction gratings and the filter wheel are coupled to a wavelength drive, particularly in a manner such that the diffraction grating of the input monochromator can be driven by means of a linear servo drive. In a further development described in German Patent No. 3,113,984, published Dec. 1, 1983, a second light source is provided and the filter wheel is constructed as a filter slide containing a planar mirror for reflecting light which is emitted by the second light source, into the path of rays of the input monochromator.
U.S. Pat. No. 4,310,244, granted Jan. 12, 1982, describes with reference to FIG. 4 therein a single monochromator containing two diffraction gratings which cooperate with a spherical mirror. A mirror which may assume two predetermined positions is located intermediate the diffraction gratings. In a first position of the mirror, light originating from the spherical mirror is led to the first diffraction grating, and in the second position of the mirror this light is directed to the second diffraction grating. The adjustment of the mirror is coupled with the wavelength drive so that the single monochromator is operated using selectively either the first or the second diffraction grating depending upon the control of the wavelength range.
The use of a double monochromator decreases the amount of stray light and increases the spectral resolution of the monochromators. But a double monochromator is attended with a loss of light. There are cases where the suppression of stray light and the increase of the spectral resolution of the monochromators are essential. But there are also cases where an increase of the light flux passing through the monochromator is desired for generating a sufficient signal by sacrificing high spectral resolution.